From Norwegian patent No. 180532 it is known a method for removing impurities, particularly boron, from molten silicon by continuous treatment of the silicon with slag. According to Norwegian patent No. 180532 the slag is added continuously or substantially continuously to the molten silicon and the slag is inactivated or removed continuously or substantially continuously as soon as equilibrium is reached between the slag and the molten silicon with respect to the impurity element or elements which are to be removed.
If the slag has a higher density than molten silicon, it is proposed in Norwegian patent No. 180532 to supply the slag to the top of the silicon bath and continuously or substantially continuously withdraw the slag from the bottom of the vessel in which the treatment is carried out.
It has however been found that it is difficult to withdraw the slag continuously or substantially continuously from the bottom of the vessel without withdrawing a substantially amount of silicon together with the slag. Further is has been found that it is difficult to remove all slag from the silicon. The yield of silicon by use of the method disclosed in Norwegian patent No. 180532 will for these reasons be low and the refined silicon can be contaminated with some slag particles.